Sen. Bernard Sanders showed no signs of ceding to Democratic presidential front-runner Hillary Clinton on Sunday, laying out a three-pronged path to swiping the nomination from her grasp as she deflects a raft of bad headlines over her emails.

The Vermont independent once again refused to use Mrs. Clinton’s unauthorized use of personal email at the State Department as a political cudgel, and he said the former secretary of state is a “significantly” better candidate than presumptive GOP nominee Donald Trump.

But he still thinks he can become the Democratic nominee in July by winning big in California and doing well enough in five other states on June 7 to nab the majority of pledged delegates. He also wants to sway superdelegates to his side in states where he won by large margins, even if those delegates rallied to Mrs. Clinton’s side before voting began.

“There are over 400 superdelegates who made a decision to vote for Secretary Clinton before anyone else was in the race,” he told NBC’s “Meet the Press.” “Before they got a sense of what the campaign was about. And all that I am saying is for those superdelegates who came onboard before I was even in the race, ‘You have got the very grave responsibility to make sure that Trump does not become elected president of the United States.’”

Mr. Sanders said failure to seize California, a huge prize, would make his mission much harder, though he pointed to polls that suggest he is more competitive against Mr. Trump than Mrs. Clinton, citing his groundswell of support from young people and working-class voters who feel marginalized by Wall Street and the so-called 1 percent.

“Democrats and progressives do well when the voter turnout is high,” he said. “Republicans do well when the voter turnout is low. I think our message of income and wealth inequality, the fact that we’re the only major country on earth not to have health care for all people is resonating. It will win the general election.”

Yet Mr. Sanders appeared caught between his disgust with Mr. Trump, who is trying to unify Republicans after his rock ‘em-sock ‘em campaign turned off segments of the GOP, and whatever leverage he has over Mrs. Clinton.

The Democratic front-runner was dealt a serious setback by a State Department inspector general’s report Wednesday that said she broke government rules by setting up her own secret email server, and that she failed to report hacking attempts and waved off warnings that she should switch to a more official email account.

Mr. Sanders said he will let an FBI probe into the situation play out instead of using as campaign talking point, though he said there is “little doubt” the Trump team will seize on the scandal.

He also refused to say whether he would accept an offer to become Mrs. Clinton’s vice-presidential pick, saying he’s focused on securing the nomination for himself and beating Mr. Trump.

“The responsibility that I accept in a very, very serious way is to do everything that I can to make sure that Donald Trump will not become elected president of the United States,” he said. “Donald Trump, for a dozen different reasons, would be a disaster as president. I will do everything that I can to make sure that does not happen.”

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